1. Field of the Invention
The present invention relates to a mask data generation method, a mask fabrication method, an exposure method, a device fabrication method, and a storage medium.
2. Description of the Related Art
An exposure apparatus is employed to fabricate, for example, a micropatterned semiconductor device such as a semiconductor memory or logic circuit by using photolithography. The exposure apparatus projects and transfers a circuit pattern formed on a mask (reticle) onto a substrate such as a wafer by a projection optical system. Along with the recent advance in the micropatterning of semiconductor devices, it is becoming necessary for an exposure apparatus to form a pattern having a line width smaller than the exposure wavelength (the wavelength of the exposure light). However, the influence of light diffraction conspicuously appears in such a micropattern, so the pattern contour (pattern shape) is not formed on the wafer intact. For example, the pattern corners are rounded or the pattern length shortens.
In recent years, to reduce deterioration in the shape precision of a pattern formed on the wafer, a mask pattern is designed by pattern shape correction processing (the so-called OPC (Optical Proximity Correction)). The OPC corrects the pattern shape in accordance with a rule-based system or a model-based system using optical simulation by taking account of the influence of the shape of each element of the mask pattern and its peripheral elements.
The model-based system using optical simulation deforms the mask pattern until a desired optical image is obtained. A method of inserting assist feature small enough not to be resolved has also been proposed.
Techniques of deriving how to insert assist patterns by numerical calculation are disclosed in Japanese Patent Laid-Open No. 2004-221594 (patent reference 1), and Robert Socha, Douglas Van Den Broeke, Stephen Hsu, J. Fung Chen, Tom Laidig, Noel Corcoran, Uwe Hollerbach, Kurt E. Wampler, Xuelong Shi, and Will Conley, “Contact Hole Reticle Optimization by Using Interference Mapping Lithography (IML™)”, Proceedings of SPIE, U.S.A., SPIE press, 2005, Vol. 5853, pp. 180-193. These techniques obtain an interference map by numerical calculation, thereby deriving a position where interference is caused on the mask and a position where interference is canceled on the mask. In the position on the interference map where interference is caused, an assist pattern is inserted so that the phase of exposure light having passed through the main pattern to be transferred and the phase of an exposure light having passed through the assist pattern are equal to each other. At the position the interference map where interference is canceled, an assist pattern is inserted so that the phase of the exposure light having passed through the main pattern and the phase of the exposure light having passed through the assist pattern have a difference of 180 degrees. As a result, the main pattern to be transferred and the assist pattern strongly interfere with each other, whereby the main pattern can be exposed successfully. The above-described interference map represents light amplitude on an image plane that is positioned in an imaging relation to a mask plane. The main pattern is an element which exists on the mask and is transferred onto the wafer.
Circuit patterns can be roughly classified into a line pattern and a contact hole pattern.
The technique disclosed in patent reference 1 calculates assist patterns by assumption that a line pattern equals a one-dimensional line and contact hole pattern equals a dimensionless point. So it cannot calculate the shape of the main pattern. To cope with this situation, the main pattern must be newly calculated after calculating, for example, the positions, shapes, and sizes of the assist patterns. It is a common practice to perform the optical proximity correction for the main pattern by calculating its specifications not from an approximate aerial image but from a non-approximate aerial image in accordance with a model-based system. In view of this, patent reference 1 requires a large number of times of calculation of non-approximate aerial images to obtain a mask pattern including the main pattern and assist patterns, which takes a long calculation time.
Also, patent reference 1 is not precision in an estimate of the interaction of the optical proximity effect between the main pattern and the assist patterns because the main pattern is assumed by a line or point in calculating the assist patterns. The corrected main pattern which is obtained later causes the optical proximity effect for the assist patterns which is obtained earlier. Consequently, a predicted effect of the assist patterns may not be obtained or the assist patterns may have an adverse effect on the obtained mask pattern. Particularly when a line pattern is used as the main pattern, assist pattern insertion is very difficult because a change of its shape due to the optical proximity correction is large at its line edge portion and curved portion.